Fix remove_and_add_spares removes drive added as spare in slot_store

Fix remove_and_add_spares removes drive added as spare in slot_store

Commit 3dedf195ea0e696f5f0bcd6f6684035095dd6aa2
introduced a regression which would remove a recently added spare via
slot_store. Revert part of the patch which touches slot_store() and add
the disk directly using pers->hot_add_disk()

Fixes: 3dedf195ea0e ("md: remove_and_add_spares() to activate specific
rdev")
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Signed-off-by: Pawel Baldysiak <pawel.baldysiak@intel.com>
Signed-off-by: NeilBrown <neilb@suse.com>